Remove outdated links to X (#2)

The links to X are not valid since the account "dan_abramov" no longer exists.

src/exercise/01.md

@@ -112,8 +112,7 @@ return <CounterContext.Provider value={value} {...props} />
 const {state, increment, decrement} = useCounter()
 ```
 
-This isn't a _bad_ solution necessarily. But
-[as my friend Dan says](https://twitter.com/dan_abramov/status/1125758606765383680):
+This isn't a _bad_ solution necessarily. But as my friend Dan says:
 
 > Helper methods are object junk that we need to recreate and compare for no
 > purpose other than superficially nicer looking syntax.
@@ -183,8 +182,7 @@ function Counter() {
 
 **This may look like overkill, and it is.** However, in some situations this
 pattern can not only help you reduce duplication, but it also
-[helps improve performance](https://twitter.com/dan_abramov/status/1125774170154065920)
-and helps you avoid mistakes in dependency lists.
+helps improve performance and helps you avoid mistakes in dependency lists.
 
 I wouldn't recommend this all the time, but sometimes it can be a help!